Conventionally known processes for synthesizing N,N-diethylaminophenols include (1) a method comprising obtaining sodium metanilate from nitrobenzene ethylating sodium metanilate with ethyl chloride or diethyl sulfate, and subjecting the ethylated compound to alkali fusion, (2) a method comprising reacting a dihydric phenol with diethylamine in the presence of an acid catalyst, and (3) a method comprising ethylating an aminophenol with ethyl chloride or diethyl sulfate in the presence of an alkali salt.
The method (1) is called an alkali fusion process. This method is of little industrial interest because it produces large quantities of waste water and sludge and also requires a lengthy process.
The method (2), in which the reaction is carried out using an excessive amount of diethylamine in the presence of an acid catalyst at a high temperature and a high pressure, provides N,N-diethylaminophenols only in a low yield with large amounts of by-products, and the excess diethylamine should be recovered. There is a further problem of corrosion of materials. Therefore, this method is very difficult to carry out on an industrial scale.
The method (3), though using an alkali salt as an acid scavenger, still involves a problem of material corrosion. Further, it is difficult to separate a by-product chloride and a produced N,N-diethylaminophenol after completion of the reaction, and purification of the desired product produces a large quantity of waste water, thus making the process considerably complicated.
In addition, while any of the methods (1), (2), and (3) inevitably requires neutralization treatment with an acid or an alkali for recovery of the produced N,N-diethylaminophenol, it is difficult, by nature of amino group-containing compounds, to judge the end point of neutralization. Moreover, the thermal instability of the N,N-diethylaminophenols has permitted of no use of distillation for purification. This naturally limits the quality of the finally obtained N,N-diethylaminophenol. More specifically, the N,N-diethylaminophenols produced by these conventional methods generally assume brown due to a small tar content (heavy content), which has often adversely affected their commercial value.
It has also been proposed that an aminophenol is subjected to reductive alkylation with acetaldehyde and the produced N,N-diethylaminophenol is recovered by distillation. The product obtained by this technique has markedly improved quality over those obtained by the methods (1) to (3). Nevertheless, the product is still unsatisfactory in quality, particularly considering that the demand of N,N-diethylaminophenols for the production of dyes for heat-sensitive or pressure-sensitive paper has recently been increasing. Thus, it has been keenly demanded to develop N,N-diethylaminophenols having higher purity and suffering from lesser coloring, depending on the end use.